Pompeii worms, Alvinella pompejana, live in tubes along hydrothermal vents on the seafloor. The worms are covered by a layer of bacteria. Scientists have discovered that the bacteria insulate the worms from the extreme heat of the vents, and the worms secrete mucus from glands on their backs to feed the bacteria. Which of the following terms best applies to the relationship between the Pompeii worms and the bacteria?